Assessing Free Radical Damage
Free radicals have developed a bad reputation, in the scientific and popular press but recent research shows how difficult a causal relationship between free radicals and disease may be to prove. Free radicals are unstable oxygen molecules that can interact with proteins, carbohydrates, lipids, and DNA, and thereby can have diverse effects on cellular function. Although some evidence points in the direction of free radicals as a culprit in a multitude of diseases, including cancer, cardiovascular disease, and Alzheimer's disease, a direct causal relationship in human beings has yet to be established. A primary reason for this is that the generation of free radicals in human beings has been difficult to measure. Free radical damage has been statistically inferred, measured by in vitro testing, and implicated in autopsy results, but never established in living humans. This has also made it "difficult to identify drugs or vitamins that may act as antioxidants to combat the effect of free radicals, and therefore, to identify appropriate doses which could be assessed in clinical trials," says Garret FitzGerald, chairman of the department of pharmacology and a professor of cardiovascular medicine at the University of Pennsylvania Medical Center, Philadelphia.
However, in a recent study, FitzGerald and his colleagues were able to noninvasively measure the effects of free radicals on the human body. The study was reported in the 1 July 1996 issue of Circulation, a journal of the American Heart Association.
Free radicals are formed by normal physiological processes, from exposures to environmental agents such as pollution, from alcohol consumption, and from cigarette smoke. The damage they do to cells is known as oxidant stress.
For a number of years, work has been done indicating that arachidonic acid--a lipid substrate found in cell membranes throughout the body--may be attacked by free radicals and oxidized and that the resulting products are detectable in the body. "We took that one step further and developed a very specific assay for one of those products so we could look with precision at its formation," said FitzGerald.
The biomarker identified by FitzGerald's team--called 8-epi prostag-landin F2 (8-epi PGF2)--is one of the isoeicosanoids, a group of 20-carbon molecules that result from free-radical attack on arachidonic acid. It is found in blood and urine. "The attraction of this [marker] is you can actually look in living human beings and get insight into the process," said FitzGerald.
The researchers examined several facets of the free radical question by studying the levels of 8-epi PGF2in the urine of 24 chronic cigarette smokers.
One endpoint investigated was the relationship between levels of the biomarker and the number of cigarettes smoked, to see that the signal diminished when people quit smoking and switched to nicotine patches. Within that context, said FitzGerald, "We thought we'd get some preliminary information on whether antioxidant vitamins might also depress this marker, which one would suspect they might."
The reason scientists would expect this effect is because the body uses antioxidant compounds found in fruits and vegetables and supplemental vitamins such as C and E to neutralize the effects of free radicals. Excessive free radical generation tends to deplete the body's antioxidant levels, so that smokers tend to have lower vitamin C levels than nonsmokers. The study confirmed FitzGerald's suspicions. According to the paper, "Excretion [of the marker] is dose-dependently increased in apparently healthy chronic cigarette smokers and falls when they switch to nicotine patches." The researchers also found a direct relationship between the number of cigarettes smoked and higher marker levels.
To measure the effects of vitamins C and E on free radical cellular damage, both vitamins were administered individually and in combination to the test subjects. After the chronic smokers took 2 g of vitamin C daily for 5 days, an average 29% decline in the level of 8-epi PGF2 was observed in their urine. Vitamin E by itself was not effective, when 800 IU of vitamin E were given along with 2 g of vitamin C daily for 5 days, an average 23% decline in urinary levels of the marker was found. However, noted FitzGerald, "We've [since] done studies extended in time and over different dose ranges, and have certainly shown that vitamin E will depress this marker as well."
FitzGerald and his colleagues are currently working to determine optimum doses of vitamins to be used as antioxidants, and are studying other drugs thought to be antioxidants as well.
Last Update: March 5, 1997